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Lipids: Source of Static Electricity of Regenerative Natural Substances and Nondestructive Energy Harvesting

Dong Wook Kim, Sang-Woo Kim, and Unyong Jeong*

D. W. Kim, Prof. U. JeongDepartment of Materials Science and EngineeringPohang University of Science and Technology (POSTECH)77 Cheongam-Ro, Nam-Gu, Pohang Gyeongbuk 37673, Republic of KoreaE-mail: [email protected]. S.-W. KimSchool of Advanced Materials Science and EngineeringSKKU Advanced Institute of Nanotechnology (SAINT)Sungkyunkwan University (SKKU)Cheoncheon-dong 300, Suwon 440-746, Republic of Korea

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201804949.

DOI: 10.1002/adma.201804949

Since the contact electrification is the charge separation occurring at the inter-face of two contact materials, surface materials (atoms, molecules) and their structural arrangement are the main factors determining the triboelectric prop-erties.[8,9] The surfaces of natural regen-erative substances (skin, hair, leaves, cells) are covered with self-assembled lipids[10] (Figure 1a). The lipid layers prevent the loss of moisture, and protect against path-ogens and ultraviolet (UV) damage. They consist of long straight hydrocarbons and their derivatives with various functional groups such as alcohols, esters, ketones, and aldehydes. Animal skin is covered with stratum corneum lipid matrix con-sisting of ceramides, cholesterol, and fatty acids.[11] Hair is coated with lipids called 18-methylicosanoic acid,[12] and cell membranes are constructed with a lipid

bilayer.[13] Lipid crystals (called “wax”) covering the outer sur-face of leaves are a mixture of lipids with different molecular lengths and functional groups bound at different positions in the lipid molecules.[14] Depending on the combination of the mixtures, these lipid crystals can take various forms.[15]

This study reveals for the first time that the lipids on the regenerative natural substances are the cause of positive contact-electrification and that most surfaces covered with a lipid layer have a similar surface potential and triboelectric output perfor-mance. Through the systematic investigation, we confirmed that not only the lipid layers on natural substances (hair, skin, leaves, cells) but also synthetic lipids show same triboelectric effects and take a higher ranking than nylon 6 in the positive triboelectric series. We create long-term usable and nondestruc-tive triboelectric nanogenerators (TENGs) by collecting the lipid layer of living leaves that are regenerated within hours. Also, we propose a triboelectric energy-harvesting vine that produces electricity from living trees while the leaves made contacts with the vine. This study opens up a new way of eco-friendly energy harvesting with preserving the nature as it is.

We compared the contact electrification of the lipids them-selves without the effect of other materials in the natural substances. Plant leaves are ideal for obtaining lipids from their surfaces because their lipids are regenerate within a few hours.[16,17] The top surface of plant leaves is covered with soluble epicuticular lipid crystals and below is the cuticle layer. The cuticle layer is composed of an insoluble polyester cutic-ular membrane impregnated in the soluble intracuticular lipid crystals.[18] To measure the surface potentials of plant leaves,

It is familiar to everyone that human skin and hair easily lose electrons and cause static electricity as they undergo friction with other materials. Such natural regenerative substances take a high ranking in the triboelectric series. Even though the static electricity of regenerative natural substances has been a long-term curiosity in human history, it is not yet clear which of their components causes the positive static charges. This study reveals that lipid layers on the surface of regenerative substances (skin, hair, leaves, cells) and even synthetic lipids are responsible for this positive static electricity and shows that it is possible to manufacture lipid-based triboelectric nanogenerators (TENGs). Using the characteristic that lipids on leaves regenerate within a few hours, lipids from living tree leaves are collected, and lipid-based nondestructive TENGs are fabricated. The concept of energy-harvesting vines is also presented, which can generate electricity when they are wrapped loosely on living tree branches. This study suggests how to harvest electricity while preserving nature as it is.

Lipid Layers

The triboelectric effect is the contact-induced electrification between two surfaces. Over the past five years, we have wit-nessed a remarkable progress in the triboelectric power gen-eration due to its high power conversion efficiency, large areal power density, low manufacturing cost, light weight, and a wide choice of materials.[1–4] Since Johan Carl Wilcke published the first triboelectric series in 1757, it has been well known that human skin and hair easily lose electrons and become positively charged as they contact or friction with other mate-rials.[5,6] Very recently, Wang and co-workers reported that plant leaves are also positively charged and they demonstrated a tri-boelectric nanogenerator (TENG) using leaves.[7] Despite the fact that these natural substances have a very high ranking in the triboelectric series, the reason is not yet clearly understood.

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the epicuticular lipid layers in various leaves (Prunus serrulata, Ginkgo biloba, Nelumbo nucifera, Menispermum dauricum) were collected on the surface of an adhesive tape by contacting the tape on the leaves and exfoliating the lipids. This transfer of the epi-cuticular lipids is often used in botanical studies.[19] In order to reveal the genuine role of the lipids in the contact electrification, we dissolved all the lipids both in the epicuticle and the cuticle of the leaves by immersing them in chloroform for 30 s,[19] and compared their contact electrification with that of the intact natural leaves. Synthetic lipids were also tested for comparison. Four different synthetic lipids (1-octacosane, 1-octacosanol, 1-docosanol, 1-docosanoic acid) were coated on a glass substrate and then transferred to the adhesive tape. Figure 1b shows the scanning electron microscope (SEM) images of N. nucifera leaf before and after the transfer of the epicuticular lipids to the adhesive tape. The lipid crystals on the outer surface were removed after the transfer process. We checked whether

the transferred leaf lipids have the same chemical structure with the lipids on the natural leaves by using the attenuated total reflection (ATR) in Fourier-transform infrared spectroscopy (FT-IR) and the grazing-incidence wide-angle X-ray scattering (GI-WAXS) (Figure S1, Supporting Information). Both measure-ments can provide only the information of the surface materials. The ATR and GI-WAXS spectra for two leaves indicate that chem-ical structure and crystal structure of the transferred lipid are the same with the lipid on the corresponding leaf. Similar analysis were reported in previous study.[20–22] The lipid exfoliation using the adhesive tape was performed only once on one leaf and the exfoliation was repeated using 10–15 clean and dried leaves. ATR and GI-WAXS measurements confirmed that the material transferred to the adhesive tape was the epicuticular lipid. After repeated transfer, the surface of the adhesive tape was com-pletely covered by the epicuticular lipids, as shown in the tilted cross-sectional SEM image in Figure 1c (upper image). When investigated with SEM and optical microscope, the surface of the transferred-lipid did not contain dirt and dust. The same multi ple transfer process was applied to 1-octacosane coated on the glass substrates, and full coverage of the lipids on the adhesive tape was accomplished (lower image in Figure 1c). SEM images of other leaves and synthetic lipids transferred to adhesive tapes are shown in Figures S2 and S3 in the Supporting Information. Once the adhesive tapes are covered with the lipids they had no stickiness (Figure S4, Supporting Information). SEM images of lipid-dissolved leaves are shown in Figure S5 in the Supporting Information. Pig skin (purchased from a local market), human hair, natural leaves were also adhered to the adhesive tape and freeze-dried retinal pigment epithelial (RPE) cells were grown on the glass substrate (Figure S6, Supporting Information).

The surface potential relates to charge trapping in dielectrics. The map of the surface potential gives information about the electronic state of the local structures on the surface.[23] The surface potential distributions and contact potential differ-ence (VCPD) can be obtained by Kelvin probe force microscopy (KPFM).[24] The VCPD between the tip and sample is defined as:

VCPD = e

tip sampleϕ ϕ−−

, where ϕsample and ϕtip are the work functions

of the sample and the tip coated with Pt, and e is the electronic charge (1.60 × 10−19 C). A positive VCPD means that electrons tunnel from the sample surface to the tip so that the sample surface is easy to be positively electrified by contact, and a nega-tive VCPD means vice versa.[23] Two contact surfaces with a large difference in VCPD result in a large triboelectric charge separa-tion.[25,26] Figure 2a and Figure S7 (Supporting Information) exhibit the potential distributions of various surfaces. The VCPD of natural substances were 1.35, 1.26, and 1.28 V for N. nucifera leaf, pig skin, and RPE cells, respectively. The VCPD of the trans-ferred leaves and synthetic lipids were in a similar range of 1.22–1.34 V. It is noteworthy that the VCPD of the bare adhe-sive tape was −4.12 V, hence the positive potentials measured in the lipids were not caused by the adhesive tape. Nylon 6 that are often used as a positive surface had a lower VCPD (0.93 V). The errors of VCPD values were small, within 5% (Figure S8, Supporting Information). The difference in CPD between lipids and nylon 6 was 0.33–0.42 V. From the previous studies which compared the CPD and charge separation,[25–27] this difference in CPD is large enough to explain that nylon 6 is less positively

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Figure 1. Lipid layers on the natural substances and their transfer to an adhesive tape. a) Natural regenerative substances covered with self-assembled lipid layers. b) Scanning electron microscopy (SEM) images showing the surface of N. nucifera leaf before and after exfoliating the epicuticular lipids. c) Tilted-view SEM images of the epicuticular lipids of N. nucifera leaf and the synthetic lipid (1-octacosane) transferred onto an adhesive tape.



charged than the lipids. The lipid-dissolved leaves had a lower VCPD (1.04–1.13 V) than the natural leaves, but the difference was not large. This small difference even after dissolving the surface lipids is because the remaining cuticular membrane is a polyester matrix composed of inter-esterified lipids[18] so that the lipid components still cause the positive contact electri-fication even though the ester bonds and epoxide bonds in the matrix are on the negative side of the triboelectric series.[2] These results indicate that the lipids or lipid-containing mate-rials are more electron donating than nylon 6.

On the basis of the results in Figure 2a, it is anticipated that the materials covered with lipids are expected to be positively electrified by contacting with a negative triboelectric material. Figure 2b and Figure S9 (Supporting Information) exhibit the surface potential (ϕs) change of the lipid-covered substances after contact electrified with poly(tertrafluoroethylene) (PTFE) that is typically used as a negative triboelectric surface. The lipids, including the RPE cells, were tightly bound to the adhe-sive during hundreds contacts with a PTFE film (Figure S10, Supporting Information). After 100 contacts with the PTFE film at 2.0 kgf, ϕs of the natural substances, the transferred leaf lipids, and the synthetic lipids increased to 700–750 V. Without further contacts, their ϕs slowly decreased and remained at ≈450 V in 7 min. ϕs of the lipid-dissolved N. nucifera leaf was ≈512 V after 100 contacts and decreased to ≈180 V in 7 min. The decrease in ϕs of the lipid-dissolved leaf was consistent with the decrease in VCPD. For comparison, ϕs of nylon 6 was 256 V after 100 contacts and decreased to 88 V in 7 min. It is well known that most materials charge positive after tribo-contact with PTFE or silicone rubber; however, most materials after tribo-contact with nylon 6 charge negative because nylon 6 is a highly positive triboelectric material. When the lipids had contacts with nylon 6, they had positive surfaces (Figure S11, Supporting Information), which infers that the lipids are more positive surface than nylon 6. These results indicate that mate-rials covered with lipids take a higher ranking than nylon 6 in

the positive triboelectric series (Figure 2c). The lipid layer on the surface of natural substances consists of various lipids with different functional groups. To avoid this chemical complexity problem, we tested the four synthetic lipids. The natural lipids are mixtures of those lipids with different molecular weight and different position of the functional groups. All the syn-thetic lipids and the natural lipids exhibited similar triboelectric characteristics, which indicates that lipids, regardless of chem-ical structures, are responsible for the positive static electricity of the natural substances.

We investigated the output performance of the lipid-based TENGs (2.5 cm × 7.5 cm) by contacting the lipid-covered sub-stances with a silicone rubber (Figure 3a; Figure S12, Sup-porting Information). The natural substances and the synthetic lipids had similar output currents (50–65 µA) and voltages (200–220 V) that are larger than those of nylon 6 (14 µA, 92 V). It is noteworthy that the output values from the natural sub-stances showed a similar trend when a PTFE film was used instead of the silicone rubber and the abaxial sides and the adaxial sides of the leaves had the similar output values even though their surface morphologies are different (Figures S13 and S14, Supporting Information). We integrated the measured current with time to calculate the transferred charge during a single contact separation cycle (Figure S15, Supporting Infor-mation). The average transferred charge was 19.6 nC when the lipid-covered substances contacted with a silicone rubber. It is also notable that the adhesion of the lipids to the adhesive tape was strong enough so that the lipids could not move over to the silicone rubber during the repeated contacts. It was confirmed by the results that TENG outputs between the silicone rubber and an Al film were the same regardless of whether using a fresh rubber or using a rubber after 7200 contacts with var-ious lipids (Figure S16, Supporting Information). The output performances of the N. nucifera leaf in different sample states are compared in Figure 3b. The lipid-dissolved leaf had a peak output current (≈32 µA) which is half of the output current of

Adv. Mater. 2018, 1804949

Figure 2. Surface charge distributions and contact electrifications of the lipid-covered substances. a) Surface charge potential distributions (3 µm × 3 µm) and the average contact potential differences (CPDs) of various material surfaces noted in each image. b) Temporal surface potentials of the materials obtained after 100 contacts with a poly(tetrafluoroethylene) (PTFE) film. c) Triboelectric series of the natural regenerative substances (skin, hair, cell, leaf).



the natural leaf. The peak current of the leaf after the epicu-ticular lipids were transferred to the adhesive tape was ≈52 µA. These difference in the output performance is consistent with the results in ϕs in Figure 2b. The triboelectric charge genera-tion occurs at the top contact surface. Surface molecules and surface topology are the key determinants of the output perfor-mance. The lipid-dissolved leaf and the lipid-transferred leaf showed no difference in the surface topology as shown in the inset images of Figure 3b and Figures S2, S5, and S17 in the Supporting Information. Since the polyester cuticular mem-brane located below the top surface lipid layer is less positively charged than the lipid layer (Figure 2a), the relative surface area between the lipid and the polyester cuticular membrane deter-mines the output performance. The results in Figure 3b indi-cate that the less performance of the lipid-dissolved leaves was due to the removal of the surface lipid, not by topological differ-ence. Because the leaf lipids are self-regenerated in a few hours after being exfoliated,[16,17] use of the transferred leaf lipids is nondestructive to living plants. Figure 3c compares the long-term stability of the TENGs made of pig skin, N. nucifera leaf, and its transferred leaf lipids. A silicone rubber coated on an aluminum electrode was used as the contact surface. The lipids were transferred to a nonconducting double-sided tape to make

a single electrode-TENG (S-TENG), whereas they were trans-ferred to a conductive double-sided tape to fabricate a double electrode-TENG (D-TENG). All the TENGs made from the transferred lipids showed similar output currents regardless of the species, seasonal conditions, and the sides of the leaves (Figure S18, Supporting Information). They remained the same performance during the whole test period (30 d), whereas the TENGs made of the natural leaves and pig skin were degraded within a few days because of drying and decomposition. The D-TENG made of the transferred leaf lipids quickly charged a capacitor (2.2 µF, 50 V) within 30 s and powered a calculator for 15 s (Figure 3d; Movie S1, Supporting Information). It instanta-neously lightened 300 light-emitting diodes (LEDs, 6500 cd m−2 per each) by hand tapping (Figure 3d; Movie S2, Supporting Information).

Natural leaves taken from grass and trees can dry out in a few days so that they cannot be used for a long time, but the leaves on living plants can be a source of long-term power gen-eration (evergreen trees can be permanent energy sources). We harvested electricity from the trees using a S-TENG, named energy-harvesting vine. Figure 4a shows a camera image of the energy-harvesting vines wrapping around branches of G. biloba. The vine (30 cm × 2.5 cm) has a multilayer structure of silicone

Adv. Mater. 2018, 1804949

Figure 3. Output performance of the lipid-based triboelectric nanogenerators (TENGs). a) Triboelectric output currents of the lipid-covered substances obtained by contacts with a silicone rubber. b) The output currents of the N. nucifera leaf in different sample states (natural leaf, the leaf after removing the epicuticular lipids, the leaf after dissolving the surface lipids). The inset SEM images correspond to the sample states. c) Long-term stability of the TENGs made of pig skin, natural N. nucifera leaf, transferred epicuticular lipids of N. nucifera leaf in the single electrode TENGs (S-TENGs) and the double-electrode TENGs (D-TENGs). d) Voltage change while charging a capacitor by the D-TENG made of the transferred leaf lipids. Insets are photographs of a calculator and 300 light-emitting diodes (LEDs) driven by the D-TENG.



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rubber/conductive fabric/silicone rubber (Figure 4b). When the leaves were shaking in the wind, electricity was produced by their contacts with the vines. The working mechanism of energy-harvesting vine is shown in Figure S19 in the Sup-porting Information. Although rain and humidity are expected to decrease the output power of the vine, fierce wind improved the performance of the vine. Figure 4c shows the results with one vine at various wind speeds. As the wind speed increased from 3.5 to 10 m s−1, the output current increased from 1.8 to 12 µA. This wind speed corresponds to a soft, fresh wind (3 to 5 steps on the scale of Beaufort wind) that feels good in daily life.[28] As the number of vines increased at a fixed wind speed of 10 m s−1, the current peak became denser and the output current increased (Figure 4d). The output power from four vines was optimized by connecting a resistor as an external load. The instantaneous maximum power was 3.97 W at 10 m s−1 wind speed (Figure S20, Supporting Information). Four energy-harvesting vines charged a capacitor (2.2 µF, 50 V) through the equivalent circuit depicted in Figure 4e and reached 5.6 V in 50 s. The charged capacitor repeatedly light-ened three high-intensity LED bulbs (48000 cd m−2 per each) for 2 min (Figure 4f; Movie S3, Supporting Information).

In summary, this study reveals for the first time that the highly positive static electricity of regenerative natural sub-stances (skin, hair, cell, plant leaves) is caused by the lipid layer on their surfaces. Not only the natural substances but also the lipids collected from the natural leaves have the same tribo-electric effect. On the contrary, the leaves whose cuticle lipids were dissolved out had much lower positive triboelectricity. When the substrates are coated with synthetic lipids, they had the same triboelectric effect as the natural lipids. On the basis of these observations, we have made long-term usable and environmentally nondestructive triboelectric nanogenera-tors by exfoliating the regenerative lipids from the living plant

leaves. In addition, we have proposed a new concept, energy harvesting vines, that can harvest electricity by simply wrapping them on tree branches. Using the lipids of regenerative natural substances as the energy-harvesting sources paves a way to pro-duce electricity with preserving the nature as it is.

Although this study focused to reveal that lipids are respon-sible for the large positive static charge in regenerative natural substances, the origin of the charge generation should be investigated. From the experimental results, different functional groups do not have a significant effect on the output performance. The long alkyl chains are likely to cause the tribo-electric effect. However, since the crystalline hydrocarbon poly-mers such as polyethylene have weakly positive surfaces, the origin of the positive charge generation in the lipid cannot be accounted for by the chemical structure alone. Precise in situ analysis of the crystal structure changes or molecular deforma-tion during the contact cycles should be thoroughly investigated with contribution of the chemical structure.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis research was supported partly by the Center for Advanced Soft Electronics funded by the Ministry of Education, Science and Technology as a “Global Frontier Project” (CASE-2015M3A6A5072945) and by the Korea Research Institute of Chemical Technology (KRICT). The human hair used in this study belonged to one of the authors, and was donated to the study following a haircut at a barbershop. Pig skin samples were acquired from a local market.

Figure 4. Nondestructive power generation with energy-harvesting vines. a) Photograph of the energy-harvesting vines wrapped on a branch of G. biloba tree. b) Cross-sectional SEM image of the vine. c) Change in the output currents generated with one vine at different wind speeds. d) Change in the output currents generated by a different number of the vines at a fixed wind speed of 10 m s−1. e) Circuit diagram and photograph of three high-intensity LEDs powered by the energy-harvesting vines. f) Charging curve in a capacitor connected to the LEDs powered by four energy-harvesting vines.



Adv. Mater. 2018, 1804949

Conflict of InterestThe authors declare no conflict of interest.

Keywordscontact electrification, energy harvesting, lipids, triboelectric nanogenerators, triboelectricity of natural substances

Received: July 31, 2018Revised: September 27, 2018

Published online:

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Lipids: Source of Static Electricity of Regenerative ...nesel.skku.edu/paper files/225.pdf · natural regenerative substances take a high ranking in the triboelectric series. Even